AD648
INSTRUMENTATION AMPLIFIER
The AD648J’s maximum input current of 20 pA per amplifier
makes it an excellent building block for the high input imped-
ance instrumentation amplifier shown in Figure 29. Total cur-
rent drain for this circuit is under 600
µA.
This configuration is
optimal for conditioning differential voltages from high imped-
ance sources.
The overall gain of the circuit is controlled by R
G
, resulting in
the following transfer function:
V
OUT
(R3
+
R4)
=
1
+
V
IN
R
G
Gains of 1 to 100 can be accommodated with gain nonlinearities
of less than 0.01%. The maximum input current is 30 pA over
the common-mode range, with a common-mode impedance of
over 1
×
10
12
Ω.
The capacitors C1, C2, C3 and C4 compensate
for peaking in the gain over frequency which is caused by input
capacitance.
To calibrate this circuit, first adjust trimmer R1 for common-
mode rejection with +10 volts dc applied to the input pins.
Next, adjust R2 for zero offset at V
OUT
with both inputs
grounded. Trim the circuit a second time for optimal
performance.
The –3 dB small signal bandwidth for this low power instru-
mentation amplifier is 700 kHz for a gain of 1 and 10 kHz for a
gain of 100. The typical output slew rate is 1.8 V/µs.
Figure 29. Low Power Instrumentation Amplifier
REV. C
–9–
AD [ ANALOG DEVICES ]
